1. Field of the Invention
The present invention relates to stand apparatus having vertically movable arms for supporting medical instruments such as ophthalmic stands, operating microscopes and roentgenographic equipments. More particularly, the present invention pertains to braking mechanisms for such vertically movable arms.
2. Description of the Prior Art
Ophthalmic stands are broadly used in ophthalmic hospitals, spectacle shops and so forth as an apparatus which suspends or mounts an ophthalmic instrument such as a refractor, refractometer or the like to locate such instrument at any desired positions with respect to a patient so that required tests can be performed. Such ophthalmic stands usually have vertically movable arm mechanisms including articulated arms for carrying heavy ophthalmic instruments such as refractometer, ophthalmometer and so forth. Among various types of such arms, the most popular one is of the balancing-weight type including a balancing weight which counterbalances the sum of the weight of the arm and the ophthalmic instrument carried by the arm.
Hitherto, two types of braking mechanism have been proposed and used for braking the arms for holding the latter at desired vertical positions. The braking mechanisms of the first type called "pedal-type braking mechanism" has a braking member which is incorporated in the arm mechanism and adapted to clamp, as a pedal is depressed, a guide rail along which the arm is moved vertically. The other type is the "solenoid-type braking mechanism" in which, as disclosed in Japanese Utility Model Laid-Open No. 56-118604, a braking member having an opening substantially of the same diameter as a guide rail is fitted around the guide rail with its one end held rotatably and, as a solenoid is energized, the braking member is attracted to contact at its inner surface with the guide rail thereby to prevent the liftable arm from moving vertically.
It should be noted that the pedal type braking mechanism is disadvantageous in that, since the pedal is mounted on the arm mechanism as a unit, the pedal is moved vertically as the arm is vertically moved so that it becomes quite difficult to depress the pedal by foot particularly when the pedal is at an elevated position. Usually, the arm mechanism comprises articulated and elongated arm and the ophthalmic instrument is carried at the free end of this arm, so that it has often been experienced that the pedal is spaced away from the position of the operator who manipulates the ophthalmic instrument. In such a case, the operator is obliged to move to a position where he can actuate the pedal.
The solenoid type braking mechanism is also disadvantageous in that it requires substantial electric power because the solenoid has to be energized during the operation of the ophthalmic instrument which takes much longer time than the movement of the arm. Further inconveniency is that when the ophthalmic stand is not used, the main switch of the instrument is turned off so that the brake also becomes inoperative. In this state, if the arm is of the balancing-weight type mentioned before, the arm is self-balanced and maintained stationary. In this condition, there may be a danger in that someone leans on the arm and falls down without being rigidly supported by the arm.